This invention relates to video signal processing systems, especially though not exclusively for producing so called production effects in television.
In our co-pending Patent Application No. 8306789 there is described a video signal processing system in which production effects are produced by writing input video signals, received in television raster format, into storage locations of a frame store so chosen that when the signals are subsequently read from the storage locations in television raster format, the signals are re-arranged in the raster to alter the image in shape, size or position, or in some other way. In order to achieve the selection of storage locations for the input signals a shape store is provided containing, at positions which correspond to respective pixel positions in the television raster, address signals identifying storage positions in the frame store at which the input video signals should be written in order to achieve the desired effect. A set of address signals describing a desired shape or other characteristic of an image is called an address map. Normally the address map is only coarsely sub-divided, comprising, say, an address for each eighth pixel, in each eighth line of a frame. Sequences of address maps are provided to produce changing effects, successive maps in a sequence describing, say every fourth field. The addresses are thus distributed on a coarse grid, and interpolating means are provided to produce addresses for the intervening pixels and also for the intervening fields. This allows addresses to be read at a rate less than `real time`, but to be used after interpolating for writing the input video signals in the frame store in real time.
The stored maps are entered in the shape store from a computer having a keyboard or other control means by which desired production effects can be entered into the computer. The computer is arranged to generate the addresses for desired maps under software control. Effects which can be produced in the image include such effects as rolling a sheet into a cylinder, turning pages of a book, or converting, say a map in Mercator projection into a representation of the map on a globe. To enable effects to be created which involve changing the image of a two dimensional object into a three dimensional object, or turning an object in space, the computer is arranged to generate addresses in three dimensions, and then convert the three dimensional addresses into two dimensional addresses by projection on the image plane, taking account of perspective.
Each storage location in the frame store in which the re-arranged video signals are written will correspond to a pixel in the output signal television raster. However, in general, the address calculated by the computer will not coincide with a storage location in the frame store but will lie within a rectangular area defined by four storage locations. Therefore on writing an input video signal (relating to a pixel in the input signal raster) in the frame store, it is usually necessary to distribute the signal among four storage locations by interpolation, (this interpolation being different from the address interpolation previously referred to). This means that up to four storage locations have to be accessed for each address obtained directly or by interpolation from the map. As a corollary, each storage location has to be accessed a plurality of times in response to different addresses. Furthermore the distribution of addresses in a map over the area of the output image may vary, so that the `density` of addresses per unit area will differ on the image. This can lead to incorrect variations in brightness over the output image. This effect can be appreciated by considering the change of a map in Mercator projection into the image of a map on a globe, in which case all video signals representing the top of the first map would be crowded in the region of the north pole on the globe. To reduce this effect, means are provided to variably attenuate video signals, prior to writing in the frame store, in proportion to the density of addresses to produce density compensation.
The system described in the aforesaid patent application represents a powerful means of producing video effects in real time. The address maps have, however, to be produced off line, and this involves the producer in trying to determine all effects which he may desire for a particular programme or series of programmes, since he does not have the facility of creating effects during the transmission of television signals.